The present invention relates to gas turbines. More specifically, the present invention relates to a system and method for capturing heat rejected from the portion of the compressor discharge air that is used to cool the turbine section of the gas turbine by transferring the heat to a fluid to be injected into the combustion section, such as gaseous fuel.
A gas turbine is comprised of three main components: a compressor section in which air is compressed, a combustion section in which the compressed air is heated by burning fuel and a turbine section in which the hot compressed gas from the combustion section is expanded. To achieve maximum power output of the gas turbine, it is desirable to heat the gas flowing through the combustion section to as high a temperature as feasible. Consequently, the components in the turbine section exposed to the hot gas must be adequately cooled so that their temperature is maintained within allowable limits.
Traditionally, this cooling is achieved by flowing relatively cool air over or within the turbine components. Since such cooling air must be pressurized to be effective, it is common practice to bleed a portion of the air discharged from the compressor section and divert it to the turbine components for cooling purposes. Although the cooling air eventually mixes with the hot gas expanding in the turbine, since it bypasses the combustion process much of the work expended in compressing the cooling air is not recovered in the expansion process. Consequently, to maximize the power output and efficiency of the gas turbine, it is desirable to minimize the quantity of cooling air used.
Unfortunately, as a result of the temperature rise which accompanies the rise in pressure in the compressor, the air bled from the compressor is relatively hot--i.e., 315.degree.-425.degree. C. (600.degree.-800.degree. F.) depending on the compression ratio. Consequently, the air bled from the compressor must often be cooled to ensure that its temperature is low enough to adequately cool the turbine components. Moreover, as is well known in the art, the quantity of air bled from the compressor for cooling purposes can be reduced by cooling the air prior to directing it to the turbine components, thereby increasing its capacity to absorb heat.
In the past, an air-to-air cooler was often used to cool the cooling air. In this arrangement, the air bled from the compressor flows through finned tubes over which ambient air is forced by motor driven fans, thereby transferring heat from the compressed air to the atmosphere. Although this method achieves adequate cooling, it detracts from the efficiency of the gas turbine since the heat energy associated with the work expended to compress the cooling air is lost to atmosphere.
It is therefore desirable to provide a system and method for cooling the air bled from the compressor for cooling purposes in which the heat removed from the cooling air is returned to the cycle.